THE ISOPODA 205 



In many Sphaeromidae one or both rami of the last two pairs are 



transversely plicated so as to increase the respiratory surface. In 



some members of these families the exopodite of the fifth pair is 



more or less indurated and forms a kind of operculum protecting 



the more delicate appendages behind it. Opercular structures 



having a similar function are formed in different ways in other 



families. In some Anthuridae the first pair of pleopods are 



enlarged and cover the remaining pairs. In the Asellidae and 



Stenetriidae the exopodite of the third 



pair performs the same function. In 



the Parasellidae an unpaired plate 



formed by the coalescence of the first 



pair forms the operculum in the female, 



the male having a tripartite operculum 



formed by the first and second pairs. 



In the Valvifera (Fig. 125), finally, 



the pleopods are covered in by the 



valve -like uropods. A very special 



line of modification has been followed 



in the case of the Oniscoidea. In the 



Ligiidae, which are in many ways the 



most primitive family, the pleopods are 



. ., .,, ,, ,. Under-side of abdomen of male 



all similar, with the exopodites stouter 7<io*mtaZOT,withooeoftLenropods 



than the endopodites but sharing in 



the respiratory function. In many telsonic segment comprising the last 



ft . . f ... , J three abdominal somites coalesced 



genera of the remaining families, how- with the teison ; j>/p, pieoinxis ; r, 

 ever, the exopodites of the first and ^,&J3?tE?t J 5'53 

 second, and sometimes of all five, pairs 



are specially adapted for aerial respiration by the development 

 within them of small cavities opening to the sxterior by slit-like 

 apertures and giving rise internally to a system of ramifying 

 tubules filled with air (Fig. 126). These tubules, which are lined 

 with a delicate chitinous cuticle, are known as pseudo-traclieae (/?). 

 In certain cases (Oniscus) in which pseudo-tracheae are absent, their 

 place is taken by a system of air-filled spaces immediately under 

 the cuticle of the exopodite. These spaces do not communicate 

 with the exterior, and appear to become filled with air by diffusion 

 through the cuticle. 



In the majority of Isopoda the second, and sometimes also the 

 first, pair of pleopods present special modifications in the male sex, 

 the only exceptions being the Epicaridea and the small and 

 aberrant family of the Gnathiidae, among the Flabellifera, where 

 no such modification is found. In the remaining families of the 

 Flabellifera and in the Phreatoicidea and Valvifera, the lamellar 

 endopodite of the second pair bears, in the male sex, a rod-like 

 process (appendix masculina) (Fig. 127, m) articulated with its inner 



